# Vision: a real-time microkernel

danos is aiming to be a **real-time operating system built on a microkernel** —
where drivers and services run isolated in user space for maximum stability, and
scheduling gives real guarantees about timing. This page is the north star: the
*why* that shapes every design decision below it. Read it before adding anything
structural.

## Microkernel

The kernel stays **minimal** — only what genuinely must run in privileged mode:

- scheduling,
- inter-process communication (IPC),
- memory management (address spaces, page tables),
- low-level interrupt dispatch.

Everything else — device drivers, filesystems, the network stack — runs as an
**isolated user-space server**, each in its own address space with only the
privileges it needs.

The payoff is **stability through isolation**. A driver bug can't corrupt the
kernel or another driver; a crashing service is contained and can be restarted,
while the rest of the system keeps running. That's the opposite of a monolithic
kernel, where a single driver fault can take everything down.

The cost is that **IPC becomes the backbone**: whatever used to be a function call
across a monolithic kernel is now a message between address spaces. In a
microkernel, IPC performance essentially *is* system performance (the lesson of
L4). So IPC must be fast, and it's a first-class concern, not an afterthought.
Hardware interrupts, too, become IPC: the kernel turns an IRQ into a message to the
driver task that owns that device.

## Real-time

danos schedules **preemptively, with guarantees about quanta** — the system must
be able to promise that a task runs when it's supposed to, within bounded time.
That imposes concrete requirements:

- **Fixed-priority preemptive scheduling.** The highest-priority ready task always
  runs; a higher-priority task that becomes ready preempts a lower one immediately.
  Not round-robin (which is fair but not predictable).
- **A calibrated, deterministic clock.** Guarantees measured in "quanta" are
  meaningless on an arbitrary tick rate — real time requires a timer calibrated to
  a known frequency.
- **Bounded interrupt latency.** Interrupt-disabled sections must be short and
  bounded, so a ready high-priority task is never delayed by an unbounded kernel
  operation.
- **Deterministic kernel operations.** Scheduling decisions should be O(1) (e.g. a
  priority bitmap), not "walk a list of unknown length."
- **Priority inheritance** (once there are locks/IPC), so a high-priority task
  blocked on a resource held by a low-priority one can't be delayed indefinitely by
  a middle-priority task — bounding priority inversion.

A consequence worth stating early: the current [kernel heap](heap.md) is a
first-fit free list, which has **unbounded allocation time** and can fragment — it
is *not* real-time safe. It's fine for one-time kernel setup, but real-time paths
must pre-allocate or use a bounded (fixed-size pool) allocator. Don't allocate on a
hot real-time path.

## What this means for the roadmap

The vision reorders the obvious hobby-kernel path. Notably, **drivers are not
built into the kernel** — so an in-kernel keyboard driver would be throwaway work.
Input devices arrive later, as the *first user-space drivers*, once the machinery
to isolate them exists. The trajectory:

1. **Calibrated timer / clock** — a known-frequency, deterministic tick. The
   foundation real-time quanta rest on. *(next)*
2. **Real-time scheduler** — fixed-priority preemptive, kernel threads first:
   context switch, task struct, priority run-queue, timer-driven preemption.
3. **User mode + address-space isolation** — higher-half kernel, ring 3, per-process
   page tables. The substrate for isolated servers.
4. **IPC** — fast message passing between address spaces. The microkernel's heart.
5. **User-space drivers** — interrupts delivered as IPC, plus MMIO/port-access
   grants. The keyboard becomes the first one, validating the whole model.

## Where we are

The foundation is in place: UEFI boot, framebuffer + [serial](testing.md),
[physical frames](frame-allocator.md), [paging](paging.md) with W^X, [exceptions
and interrupts](interrupts.md), a [timer](device-interrupts.md), and a
[heap](heap.md) — plus a [test harness](testing.md). The kernel boots and has its
core services; the next milestones make it *schedule*, then *isolate*.